Required Modules in Drupal in module.inc

Function Definition: 

function drupal_required_modules() {
  return array('block', 'filter', 'node', 'system', 'user');
}

Example:

$list = drupal_required_modules() ;
ep($list);

Array ( [0] => block [1] => filter [2] => node [3] => system [4] => user )

So, here is the place the list of required core modules desided in Drupal.